Rifaximin (1) is a non-aminoglycoside semi-synthetic, nonsystemic antibiotic derived from Rifamycin, useful for the treatment of traveler's diarrhea in adults and in children 12 years of age and older caused by Escherichia coli bacteria. Rifaximin has also been evaluated for the treatment of irritable bowel syndrome, diverticular disease, hepatic encephalopathy, pyogenic skin infections, and as an antibacterial prophylactic prior to colon surgery. Chemically, Rifaximin is (2S, 16Z, 18E, 20S, 21S, 22R, 23R, 24R, 25S, 26S, 27S, 28E)-5,6,21,23,25-pentahydroxy-27-methoxy-2,4,11,16,20,22,24,26-octamethyl-2,7-(epoxypentadeca-[1,11,13]trienimino)-benzofuro[4,5-e]-pyrido[1,2-(alpha)]-benzimidazole-1, 15(2H)dione, 25-acetate.

Rifaximin is currently sold in the US under the brand name Xifaxan™ by Salix Pharmaceuticals. It is also sold in Europe under the names Spiraxin™ Zaxine™, Normix™ and Rifacol™ and in India under the name Rifagut™.
U.S. Pat. No. 4,557,866 describes a new process for the synthesis of pyrido-imidazo-rifamycins of formula I. The process comprises reacting the rifamycin O with 4-methyl-2-aminopyridine.
U.S. Pat. No. 7,045,620, U.S. Pat. No. 7,612,199, US 20080262220 and US 20080262232 disclose crystalline polymorphous forms of Rifaximin (INN) antibiotic named Rifaximin alpha and Rifaximin beta, and a poorly crystalline form named Rifaximin gamma. These forms can be obtained by means of a crystallization process carried out by hot-dissolving the raw Rifaximin in ethyl alcohol and by causing the crystallization of the product by the addition of water at a determinate temperature and for a determinate time period. The crystallization is followed by drying carried out under controlled conditions until specific water content is reached in the end product in order to consistently obtain the above mentioned homogeneous polymorphic forms of Rifaximin.
US20080262024 describes forms of Rifaximin (INN) antibiotic, such as the poorly crystalline form named Rifaximin gamma, along with the production of medicinal preparations containing Rifaximin for oral and topical use.
US 20050272754 relates to Rifaximin polymorphic forms alpha, beta and gamma, the processes for their preparation and the use thereof in the manufacture of medicinal preparations for the oral or topical route.
WO 2008155728 describes a process which enables Rifaximin in a completely amorphous form to be obtained. Said process comprises the steps of dissolving crude Rifaximin in absolute ethanol while hot and then collecting after precipitation by cooling the title compound in amorphous form.
US 20090312357 discloses amorphous Rifaximin, methods of making it, and pharmaceutical compositions containing it. Also described are methods of converting amorphous Rifaximin to crystalline Rifaximin and vice versa.
WO 2009108730 relates to Rifaximin polymorphic, salt, hydrate, and amorphous forms, to their use in medicinal preparations and to therapeutic methods using them. Form zeta, Form eta, Form alpha-dry, Form i, Form beta-1, Form beta-2, Form epsilon-dry, and amorphous forms of Rifaximin as wells a mesylate salt are described.
US 20090082558 describes a stable amorphous form of Rifaximin. This form is chemically and polymorphic stable on storage and can be prepared by dissolving Rifaximin in a solvent to form a solution which is precipitated by adding an anti-solvent and isolating of the precipitated amorphous Rifaximin as an end product.
US 20090130201 describes crystalline polymorphous forms of Rifaximin (INN) antibiotic named Rifaximin delta and Rifaximin epsilon useful in the production of medicinal preparations containing Rifaximin for oral and topical use and obtained by means of a crystallization process carried out by hot-dissolving the raw Rifaximin in ethyl alcohol and by causing the crystallization of the product by addition of water at a determinate temperature and for a determinate time period, followed by drying carried out under controlled conditions until reaching a settled water content in the end product.
US 20100010028 describes polyols which stabilize polymorphous forms of Rifaximin, in particular the beta form. When polyols having at least two hydroxyl groups are added to Rifaximin powder, polymorph beta is stable and remains stable in time independently from the environment humidity. A method to prepare formulations constituted by pure and stable polymorphous forms able to give a pharmaceutical product is also described.